On days 1 to 3 germfree rats (4 groups, 6 per sex per group) will be seeded with feces from breast fed babies. Each group will be housed in a separate positive pressure isolator. From day 1 animals will be fed the following; 1) human breast milk, 2) Enfamil(tm) milk formula, 3) Enfamil(tm) milk formula supplemented with fructooligosaccharides (FOS: 5% wt/wt) and 4) standard chow. On days 1 (pre-seeding), 5, 9 and 13 freshly voided feces from each animals will be collected and the bifidobacteria, lactobacilli and clostridian will be grown and counted. On day 17 the rats will be killed. Their hindgut contents will be removed and tested as follows: 1) pH, 2) percent water, 3) bacterial cellular fatty acids, 4) short chain fatty acids, 5) bile acid metabolism 6) neutral sterol metabolism, 7) four bacterial enzymes (beta glucosidase, beta glycosidase, azoreductase and nitroreductase) and 8) quantitative PCR probing for 10 key species, and lastly, 9) bacterial counts on a 12 selective and/or differential media. How well FOS convert a formula-fed flora to its more desirable breast-fed counterpart will be assessed by comparing the results. PROPOSED COMMERCIAL APPLICATION; Germfree rats seeded with human feces, from an infant or an adult, will be used as an in vivo system to screen any orally ingested compound, from infant milk formula to olestra, from antibiotics to fiber.